1. Field of the Invention
The present invention relates to a servovalve apparatus and, more particularly, to a servovalve apparatus of the type having a servovalve and a force motor operatively connected to a valve element reciprocatably received in a bore formed in the body of the servovalve, for reciprocating the valve element within the bore.
2. Description of the Prior Art
The servovalve apparatus of the type mentioned above is disclosed in, for example, Japanese Patent Laid-open No. 24975/78. The force motor of this known servovalve apparatus includes first and second magnet yokes which clamp therebetween an annular magnet. The first and second magnet yokes cooperate with each other to define therebetween an annular gap concentric with the annular magnet, so that the magnetic fluxes generated by the annular magnet are concentrated in the annular gap. The first magnet yoke is attached to the end of the valve body of the servovalve such that the annular gap communicates with the bore in the valve body. The second magnet yoke has therein a central bore concentric with the annular gap and extending through the second magnet yoke coaxially with the bore in the valve body.
A connecting member has a hollow cylindrical wall positioned in the annular gap concentrically therewith, and an end wall closing the axial one end of the hollow cylindrical wall adjacent to the valve body and integrally connected to the axial end of the valve element reciprocatably received in the valve body, adjacent to the above-mentioned axial end of the hollow cylindrical wall.
A cylindrical coil has a multiplicity of turns wound around the hollow cylindrical wall of the connecting member concentrically therewith. The cylindrical coil is connected to the valve element through the connecting member. Each turn of the coil has a circular cross-section. The cylindrical coil is reciprocated within the annular gap in response to the electric current passing through the coil. The reciprocating motion of the cylindrical coil is transmitted to the valve element through the connecting member.
The neutral position of the valve element in the bore of the valve body is adjustable by means of a neutral position adjuster which includes a cylindrical resilient member made of a rubber or the like and loosely received by the central bore in the second magnet yoke with an axial one end of the resilient member fixed to the surface of the end wall of the connecting member opposite to the valve element, a rod having one axial end abutting the other axial end of the cylindrical resilient member, and a feeder connected to the end of the second magnet yoke opposite to the valve element and engaging the other axial end of the rod to cause an axial movement of the rod in one and the other directions within the central bore in the second magnet yoke. Thus, the end wall of the connecting member is connected to the second magnet yoke through the cylindrical resilient member.
As the cylindrical coil is energized, the coil generates heat in accordance with the Joule's law. Most of the heat thus produced is transmitted through air layers in contact with the cylindrical coil to the first and second magnet yokes and dispersed to the atmosphere. The remaining small part of the heat, which is not transmitted to the magnet yokes, is conducted to the valve element through the connecting member and also to the second magnet yoke through the connecting member, cylindrical resilient member, rod and the feeder.
The rate of heat transmission to the first and second magnet yoke through the air layers is low. The rate of heat transmission to the valve element and cylindrical resilient member through the connecting member is also low because the end wall of the connecting member has a thin thickness. Accordingly, the Joule heat generated by the cylindrical coil is confined within the annular gap to undesirably raise the temperature of the cylindrical coil. The elevated temperature prevents the electric current passing through the cylindrical coil from being increased and, hence, it is impossible to increase the force imparted to the cylindrical coil in proportion to the electric current passing therethrough. This in turn makes it difficult to drive at high speed the valve element connected to the cylindrical coil.